Most of us are familiar with the theory of evolution -- the survival of the fittest -- but what about reversible evolution?
Can animals re-grow features they lost millions of years ago? Apparently so, based on some of the largest kangaroos ever to evolve, which resurrected crests on their teeth that were present in distant ancestors more than 20 million years ago.
New research from UK and Australian scientists has raised questions about the long-held idea that once a species loses a feature or organ, it can't be recovered in the future.
The changes in climate, diet and habitat were the drivers of change for the kangaroos, whose environment changed from forests to more arid surroundings over millions of years.
As the forests retreated towards the coastline, the species was forced to eat more grass -- which meant their teeth needed to cut rather than chomp at their food, the researchers say.
Flinders University School of Biological Sciences PhD candidate Aidan Couzens says the growth of grasses and abrasive plants forced the kangaroos to shift their diets.
He used a simple mathematical rule to show that re-evolving features might not be so hard.
"We show that small changes to a 'rule' that determines how teeth form in the embryo have allowed some kangaroos to partly turn back the clock on evolution.
"Using these rules, we can start to predict the pathways evolution can take," he says.
Biologists have often discounted the idea evolution could go the other way, believing any examples of it to be convergent evolution.
The paper's co-author associate professor Gavin Prideaux from Flinders University explains convergent evolution as instances where "similar features evolve independently in organisms that are not closely related".
But the researchers argue that bringing back "mothballed features" can happen under the right pressures in an animal's ecology.
Kangaroos and wallabies have long been studied and seen as an indicator of historic climate change because they'd been around for at least 30 million years.
"We are discovering more about how early forms were adapted to the abundant soft-leaved forest plants and how later macropods adapted to more arid conditions," Dr Prideaux says.
The research was published in Evolution this week.